The integration of a plurality of functions such as sample handling, treatment and analysis is an important aspect of present trends in the provision of integrated analysis of samples such as blood samples, for example in point of care analysis services. Microfluidic devices provide for sample handling and treatment in miniaturised integrated platforms through channels and other sample handling features with smallest dimensions of the order of less than 1 mm, more specifically less than 0.1 mm. Centrifugal microfluidic devices or “lab on a disk” devices are particularly promising as they do not require external pumps or other connections but can be designed to carry out a multitude of sample handling and treatment functions controlled purely by a sequence of rotational frequencies of the device.
For a fully integrated point of care (or other) analysis system a detection or analysis functionality must also be built into the device. One possibility is to provide an optical window to a detection zone of the device where an assay for target molecules in the sample being handled is carried out. For example, the detection zone can be provided with antibodies immobilized in the detection zone so that a target protein/antigen will bind to the immobilized antibodies to be retained in the detection zone. The immobilized target molecules can then be detected using optical detection methods, for example by detecting fluorescence if target molecules have previously been labelled with fluorescent dye.
One particularly interesting detection method is based on the Surface Plasmon Resonance (SPR) effect, which does not require the target molecules to be labelled. Briefly, the antibodies (or other probe molecules) are immobilized on a specialized detection surface (typically a thin metal surface coated onto a prism or a metal coated diffraction grating). Light from a light source such as a laser or a light emitting diode (LED) with or without spectral filtering is directed to the detection surface in a carefully controlled manner so that most of the energy of the incident light is absorbed by surface plasmons in the detection surface.
The SPR interaction is dependent on the relative refractive indices of the detection surface and its immediate surroundings. The binding of target molecules to the probes changes the refractive index in the immediate vicinity of the detection surface thereby changing the settings of the incident light beam at which SPR occurs. These settings include, for example, wavelength or angle of incidence. By detecting the light received from the detection surface, the presence of target molecules bound to probe molecules in the detection zone can be detected by a change in the intensity of received light, a change in the angle of incidence at which maximum absorption (resonance) occurs or a change of the wavelength at which this occurs. Alternative known methods detect corresponding changes related to the phase of the received light.
Microfluidic SPR based “lab on a disk” detection devices are described in WO-A-2008/057000, providing a detailed description of the target/probe binding, the SPR detection mechanism and its implementation in a “lab on a disk” microfluidic device using a grating as a detection surface.
JP2004/117048 describes a SPR detection system in a prism configuration using a rotating disk. Another rotating disk SPR detection system based on the prism configuration is described in WO/03102559. A characterization of SPR on rotating disk substrates has been provided by Fontana, E, Applied Optics 43, 79-87, (2004) and Chiu, K P et. al. Jap. J. appl. Phys. Part 1 43, 4730-4735 (2004). Detection systems using detection areas in a rotating support with other detection mechanisms are described in U.S. Pat. No. 5,994,150, US2001031503, U.S. Pat. No. 6,653,152, U.S. Pat. No. 6,277,653 and WO9721090. All references referred to above are hereby incorporated by reference herein.
Typically, a microfluidic “lab on a disk” substrate (as used herein to refer to a substrate arranged for microfluidic handling of fluids using the centrifugal force by rotating the substrate) is placed in a compact disc (CD) like reader device for controlled rotation of the substrate. The reader comprises an optical detection module with a light source and detector to detect an optical signal from a detection zone of the substrate.
The detection module receives light from the substrate while the substrate and detection module move relative to each other and can measure and store an intensity profile over the substrate surface. More detailed information can be captured and stored if acquisition is limited to the detection zone(s). To accomplish this, the detection module and detection zone must be appropriately aligned when signal acquisition occurs. This can be achieved in a number of ways, for example by mechanical coding between the substrate and a driving element of the reader so that the orientation of the substrate in the reader can be known if the position of the driving element can be measured or by providing a separate trigger mark on the substrate which can be detected by a separate trigger system to indicate that the detection zone is aligned with the detection module and trigger data acquisition. These approaches have a number of drawbacks in that the alignment between the mechanical coding or trigger mark and the detection zone must be highly accurate to provide precise triggering of data acquisition. This is particularly important in SPR detecting systems which rely on a careful alignment between the detection module and the detection zone in particular when the detection zone is small and even more so in multiplexed systems having a plurality of small detection zones. Moreover, these approaches require additional components and modifications of conventional CD like reader systems, thus increasing overall system costs.